Focused X-ray beams have shown a promising future in low-energy external beam radiation therapy. The development of X-ray lenses for diagnostic X-ray energy levels (≈100 keV) enables the use of external beam radiotherapy at these energies. Like conventional high-energy beam therapy, the shaping of the beam is a concern.
An X-ray lens may focus X-ray beams for delivery to a target, while minimizing the dose delivered to the tissues along the beams' path. FIG. 1 illustrates how a target inside a patient is radiated with an X-Ray cone. The intersection of the patient's skin and the X-Ray cone is a larger area than the area of the target. Hence, the energy per unit area (dose) is smaller outside the target area, in particular at the skin. This is due to the cone geometry of the lens system.
X-ray lenses image an X-ray source from an anode (the electron focal point) onto a focal plane. The X-ray lens's deflection of the x-ray beams is similar to an optical lens's deflection of visible light. The image on the anode is produced by the X-ray lens onto the focal plane (FIG. 2). Due to the physical limitations, as well as the characteristics of the focusing system (lens), the image of an ideal point source at the focal plane will not be a point. The image of such a point source will be determined by the characteristic function of the optics and will be represented by the system's Point Spread Function (PSF). The PSF together with the actual shape of the focal point of the electrons on the X-ray anode, which is also of finite extension, determines the size and the shape of the X-Ray focus at the focal plane. In general, the x-ray focal spot will not match the size and the shape of the target that is to be radiated, but will be considerably smaller than the target.
To remedy this problem, one solution is for the overall X-Ray system (source and the lens) to sweep the target in order to create the needed coverage. This kind of sweeping involves mechanical movement of the overall X-Ray system, such as a robotic arm. Alternatively, an MLC/port type device may shape a broad beam by electromechanical methods, but such a method is slow and unreliable. In order to create a needed flux pattern at the target, i.e. a dose distribution over the target, there is a need for multiple exposure patterns created by several overlapping sweepings.